1. Field of the Invention
The invention relates to an injection molding machine, especially for injection stamping of large-area plastic moldings, comprising at least a first injection mold which has two mold halves and which forms a first cavity with a first projected area, and a second injection mold which has two mold halves and which forms a second cavity with a second projected area, one mold half of the two injection molds respectively being fixed on a common carrier unit and for at least one of the two injection molds there being support means acting in the opening direction.
2. Description of Related Art
An injection molding machine of the above indicated type is known from German Patent Application DE 199 37 100 A1 and comprises two injection molds which each have a cavity bordered by two mold halves in the region of the parting plane. This injection molding machine has a turning means which comprises a reversing plate to which one mold half of the two injection molds is attached. In the parting planes of the two injection molds, there are four hydraulic cylinders arranged symmetrically. They produce a parallel, simultaneous short stroke when the injection molds are opened as a support for parallel driving of the respective mold. However, the arrangement is not suited for injection stamping of two components of a large-area plastic molding which are produced by means of cavities which have projected areas of different size. The two injection molds should be subject to the same clamping force in the closed state. However, for components with a smaller projected area it can be too large and can adversely affect the quality of the plastic molding to be produced.
German Patent DE 43 40 258 C2 discloses an injection molding machine with a multilevel mold which has two injection molds each of which has two mold halves and which can be closed by means of hydraulic gearing. The hydraulic gearing has double-acting cylinders which comprise pistons with action surfaces of different dimensions. By the corresponding triggering of the pistons when the injection molds are closed, force behaviors are implemented which are designed to be matched to the two injection molds.
Especially in the field of vehicle glazing, there is an effort to replace glass panes by injection moldings produced from transparent plastic. Especially a polycarbonate can be used for this purpose. This material, which is injected as a highly viscous amorphous substance into a cavity, must be processed with as little stress as possible to produce a pane in order to ensure durability when exposed to the weather. Stresses in the component could lead to an adverse effect on the protective lacquer layer to be applied to the hardened polycarbonate material.
For injection molding of thin-walled, large-area components, different processes can be used, such as the so-called expansion stamping process or the so-called parallel stamping process. In this connection, a liquid starting material is injected into a cavity of an injection molding machine which encompasses a so-called dip edge tool. Upon cooling, the material experiences processing shrinkage. Therefore, in all these processes, the cavity is overfilled by the shrinkage volume. The injected material is then cooled under a pressure of, for example, 200 bar. The associated volume reduction is balanced by one of the mold halves following until the component to be produced at the mold removal temperature has assumed its rated dimensions.
For reasons of engineering and economics, in the production of a plastic molding formed from two components, it can be advantageous to produce the two components in an injection molding machine, the first component which can be made pane-shaped being formed in a first cavity and the second component which forms, for example, the frame of the pane being formed as an back-injection of the first component in the second cavity. For reasons of material compatibility and to ensure a reliable connection between the two components, the material of the second component should correspond essentially to the material of the first component so that, when using polycarbonate or a polycarbonate blend, the back-injection should be executed according to a stamping process. In the forming of the second component, the problem is that processing should take place with as little stress as possible.
In a version of the first component as a pane body and the second component as a frame-like back-injection on the pane body, the base surface of the second component is smaller than that of the pane body. Stamping of the second component with the clamping force used for stamping the first component is not easily possible since the clamping force selected for stamping of the first component would lead to an overly great internal pressure in the cavity for the second component. The internal pressure is found from the product of the clamping force and the area of the cavity projected in the closing direction.
To produce a frame-like back-injection on a pane, in the past, a conventional injection molding process had been used in which, optionally, a hot channel system with needle seal nozzles is used and a holding pressure is applied by means of an injection unit over several gating points distributed over the cavity. In this process, there is the disadvantage that the internal mold pressures are very high and there is a nonuniform pressure distribution in the cavity depending on the distance to the gating points. The material shrinkage which takes place when the injected material is cooled can be balanced only up to the so-called sealing point which is, for example, roughly 190° C., at which the sprue channels freeze or the material core freezes.
Alternatively, a frame-like back-injection on a pane has been produced to date such that the second component necessary for the back-injection is placed via a hot channel distributor in a cavity, the cavity being overfilled by the shrinkage volume. During cooling, the injected material is stamped out by means of stamping dies which are actively pressed into the melt by means of special hydraulics with a fixed force. Since the force for the stamping dies must be actively applied and large area components, moreover, can have base areas of 0.5 m2 or more, the required force at the desired internal pressure of roughly 200 bar rises to 10000 kN or 1000 tons. This leads, on the one hand, to very massive cylinders, force transfer systems or the like for the stamping dies having to be used. On the other hand, the power required for the hydraulics must be drawn from the injection molding machine used at the time or must be produced by means of an additional assembly.